Sample analysis is used to determine a property or properties of a sample. One type of analysis is an optical analysis. Optical properties of a sample can provide information about the sample. Optical sample analysis is often used for detection in liquid chromatography. Liquid chromatography is used to separate a sample liquid mixture into its individual components and to quantify each component in the sample.
Evaporative light scattering detectors (ELSDs) are used for liquid chromatography analysis. In a typical liquid chromatography configuration, a solvent mixture (the mobile phase) containing dissolved solute (the analyte) flows continuously through a column and into an ELSD. Within the ELSD, liquid from the column is converted into small droplets by a nebulization process that uses a carrier gas, usually nitrogen. As the gas stream passes down a drift tube, the solvent evaporates and any non-volatile analyte present is ideally converted to finely dispersed solid particles that exit the drift tube and pass into the light scattering detector (LSD), which is the final section of the ELSD. When a sample is injected into an upstream end of the column, it travels slowly down the column and is separated into its individual components (the analytes). The analytes are sequentially eluted by the column. When an analyte is eluted, the liquid eluent contains analyte in addition to the mobile phase. At all other times, the liquid eluent is composed only of mobile phase.
A typical light scattering detector device (LSD) section of an ELSD is shown in FIG. 1. A light source 101 produces a collimated beam 102 that passes through apertures 103, which help to produce a well-defined beam. The light passes through a detector cell 104 and enters a light trap 105. The column eluent is converted into a gas stream, which exits a drift tube (not shown) and enters the detector cell 104 in a direction perpendicular to the plane of the paper in FIG. 1. Particles 106 within the gas stream pass through the beam 102, causing light to scatter. Some of the scattered light 107 is focused by a lens 108 onto a photodetector 109. The detector 109 is oriented to receive only scattered light; e.g., the detector axis 110 can be oriented perpendicular to beam 102, as shown in FIG. 1. The photocurrent of the detector 109 is processed by electronic circuitry (not shown) to provide the final output signal.
During operation of an ELSD equipped with an LSD such as that shown in FIG. 1, scattering from evaporated mobile phase liquids, from nebulizer carrier gas and from detector cell components produces a small signal in the detector 109, which is the background or baseline in the analysis. The light trap 105 is intended to capture the through-beam, the direct light 111 from the collimated beam 102, so that the through-beam does not reach the detector 109. If analyte particles are present, they also scatter light. Thus, analyte particles give an additional signal on top of the baseline. The total amount of light scattered by the analyte depends on both the number and size of the analyte particles that enter the light beam. A plot of detector signal versus time is called a chromatogram. A typical chromatogram exhibits multiple peaks, each corresponding to one of the eluted analytes.
A goal for ELSD units is to maximize sensitivity so the limit of detection is as low as possible. More powerful light sources and more efficient light collection optics can increase the size of the analyte signal, but the light source power is limited in commercial LSD sections because scattering from sources other than analyte particles quickly swamps the detector signal as the light source power is raised past relatively low light levels. Spurious scattering arises from sources other than the analyte particles including, for example, mobile phase components, nitrogen gas molecules, and the physical structure of the detector cell, as stated above. Conventional light traps also allow a small fraction of the light beam to be directed back toward the detector. These light traps are based on multiple reflections in which some fraction of the light is absorbed for each reflection. However, some degree of scattering also occurs during each reflection. Well-defined, e.g., collimated, light beams help reduce troublesome spurious scattering by detector cell components and the light trap at increased light source powers, but such beams may be difficult to realize with incandescent light sources.
Typically, the intensity of detected light attributable to particle scattering is only a tiny fraction (e.g., less than 10−8) of the original light beam intensity. Thus, even a very small amount of spurious scattering can produce a relatively intense background. This large background of scattered light results in a large, inconvenient DC baseline offset. The large background is also a source of noise and at high enough levels, it will saturate the detector.